Source of Acquisition NASA Johnson Space Center

Shian U. Hwu, Justin A. Dobbins, Yin-Chung Loh, Quin D. Kroll, and Catherine C. Sham

Abstract- The Space Shuttle Ultra (UHF) is planned to provide Frequency (RF) coverage for astronauts working underside of the Space Shuttle Orbiter (SSO) for thermal tile inspection and repairing. This study is to assess the Space Shuttle UHF communication performance for astronauts in the shadow region without line-of-sight (LOS) to the Space Shuttle and Space Station UHF antennas. To insure the RF coverage performance at anticipated astronaut worksites, the link margin between the UHF antennas and Extravehicular Activity (EVA) Astronauts with significant vehicle structure blockage was analyzed. A series of near-field measurements were performed using the NASNJSC Antenna test facilities. Computational investigations were also performed using the electromagnetic modeling techniques. The computer simulation tool based on the Geometrical Theory of Diffraction (GTD) was used to compute the signal strengths. The signal strength was obtained by computing the reflected and diffracted fields along the propagation paths between the transmitting and receiving antennas. Based on the results obtained in this study, RF coverage for UHF communication links was determined for the anticipated astronaut worksite in the shadow region underneath the Space Shuttle. Shian U. Hw,Justin A. Dobbins, Yin-Chung Loh, Quin D. Kroll, and Catherine C. Sham

the Space Shuttle structure obstructions. The Abstract- The Space Shuttle (UHF) (F@) signals have to overcome the vehicle structure blockage communications system is planned to provide Radio Frequency along the propagation paths. To ensure the RF coverage can 0 coverage for astronauts working underside of the Space meet the requirements, the link margin evaluation must take into Shuttle Orbiter (SSO) for thermal tile inspection and repairing. account such antenna pattern and signal attenuation This study is to assess the Space Shuttle UHF communication performance for astronauts in the shadow region without due to blockage. The system must be able to compensate for the line-of-sight (LOS) to the Space Shuttle and Space Station UHF signal attenuation, and provide adequate signal level throughout antennas. To insure the RF coverage performance at anticipated designated areas of coverage for the communication links to astronaut worksites, the link margin between the UHF antennas function effectively. and Extravehicular Activity (EVA) Astronauts with significant In shadow regions, such non-line-of-sight (NLOS) locations vehicle structure blockage was analyzed. A series of near-field measurements were performed using the NASA/JSC Anechoic will experience relatively high and signal degradation. Chamber Antenna test facilities. Computational investigations The signal degradation and path loss caused by structure were also performed using the electromagnetic modeling blockages are in general much higher for signals in the techniques. The computer simulation tool based on the frequency bands, such as LAN (2.4 and 5 Geometrical Theory of Diffraction (GTD) was used to compute the GHz), than for those used by the Space Shuttle astronaut signal strengths. The signal strength was obtained by computing communications at the UHF frequency bands (414.2 and 417.1 the reflected and diffracted fields along the propagation paths between the transmitting and receiving antennas. Based on the MHZ). results obtained in this study, RF coverage for UHF communication links was determined for the anticipated astronaut worksite in the shadow region underneath the Space Shuttle.

Index Terms-Geometrical theory of diffraction, Space shuttles, Space vehicle communication, UHF communication.

I. INTRODUCTION HE Space Shuttle Orbiter (SSO) payload bay Ultra High TFrequency (UHF) antenna is planned to provide UHF communication links for Extravehicular Activity (EVA) astronauts in the worksite underneath the Space Shuttle for thermal tile inspection and repairing, as shown in Figure 1. The payload bay antenna pattern will be blocked and distorted due to Fig. 1. Astronauts in worksite underneath Space Shuttle for thermal tile inspection and repairing. Manuscript received October 14,2004. This work was supported in part by the NASA Johnson Space Center under Contract NAS9-19100. Shian U. Hwu is with Lockheed Martin Space Operations, Houston, Texas A key goal of the work was to avoid the assumption that any 77058 USA (phone: 281-333-7559; : 281-333-7620; email: blockages of the line-of-sight 0.0s) automatically precluded [email protected]). Justin A. Dobbins is with NASA Johnson Space Center, Houston, Texas the site from having communication coverage. Instead, This 77058 USA. (e-mail: [email protected]). study is based on a detailed 3D electromagnetic modeling, Yin-Chung Loh is with Lockheed Martin Space Operations, Houston, Texas which took into account difiactions along propagation paths 77058 USA (e-mail: [email protected]). over and around blockage structures. In many cases, as Quin D. Kroll is with NASA Johnson Space Center, Houston, Texas 77058 USA. (e-mail: [email protected]). predicted by the computer simulations and supported by the Catherine C. Sham is with NASA Johnson Space Center, Houston, Texas experimental measurements, the difiactions can result in 77058 USA. (e-mail: [email protected]). adequate received signals and link margin for communication coverage. 0-7803-8812-7/05/$20.00 0 2005 IEEE The purpose of this study is to assess the signal strengths and at the UHF frequency. Thus, the Space Shuttle structures are RF coverage performance for astronauts working in the shadow modeled as perfect electrical conductors. region underneath the Space Shuttle. Both experimental The Space Shuttle payload bay and Space Station UHF measurements and computer simulations were performed. Due antennas are quadrifilar helical antennas. The operating to the large size of the Space Shuttle, full-scale vehicle mockup frequencies are 414.2 MHz and 417.1 MHz. An average measurements requiring a large outdoor range are difficult to frequency of 41 5.65 MHz was used in this analysis for the UHF implement. The scaled Space Shuttle mockup measurements in communications. Anechoic Chamber are feasible. Another approach in evaluating B. Signal Strength Computations the communication performance under vehicle blockage is using an electromagnetic modeling technique [ 1-71. Computer At high frequencies the scattering fields depend on the simulations have advantages when (1) it is too expensive and electrical and geometrical properties of the scatterer in the dangerous to perform real tests or experiments on an existing immediate neighborhood of the point of reflection and complex vehicle; (2) the vehicle is not available or cannot be diffraction. In the field computation, the incident, reflected, and tested for a real scenario. difftacted fields are determined by the field incident on the reflection or difftaction point multiplied by a dyadic reflection Using simulation tools for initial investigation before the or diffraction coefficient, a spreading factor, and a phase term. actual measurements are performed for the final configuration The reflected and diffracted field at a field point r', Fd(r'), in can reduce the project costs and time. A reliable analytical tool general have the following form: provides matrix comparison for proposed approaches and helps to find the best solutions. Modem high-speed computers with Pd(r') = P(r) PdA',~(s) e-jh . (1) large storage capacity have made possible the theoretical calculation of the signal strength with antennas mounted on where E'(r) is the field incident on the reflection or diffraction complex structures such as Space Shuttle. The main advantage point r, D',d is a dyadic reflection or diffraction coefficient, of this mathematical method is that, once the geometry of the kd(s) is a spreading factor, and s is the distance from the vehicle has been represented in the computer, the analyses of reflection or diffraction point r to the field point r'. andkd different astronaut positions can be easily evaluated. The RF can be found from the geometry of the structure at reflection or coverage analysis can be performed on the basis of a computer diffraction point r and the properties of the incident wave there. evaluation; the number of measurements can be cut down to a Thus, the total fields (Et0? can be obtained by summing up the reasonable minimum for verification and validation purpose. individual contributions of the direct field (E?, reflected field (Eref),and diffracted field (Ed1?along the propagation paths, as Since the Space Shuttle is large in terms of , the following, Geometrical Theory of Diffraction method (GTD) is a suitable candidate for the computational task. The Method of Moments (MOM) is a feasible tool that provides better accuracy. However, excessive computer resources and computing time will be required for electrically large Space Shuttle structure Emt: Total field at the observation point, models. The MOM technique is suitable to verify GTD results E& : Direct fields from antennas, with partial structure models immediate to the antenna. In this Eref: Reflected fields from plates and curve surfaces, study, the GTD was used to compute the electric fields. The E'f : Difftacted fields from plates and curve surfaces. total field was obtained by summing the direct fields from the antennas and the reflected and diffracted fields along the propagation paths between the transmitting and receiving antennas. The computed signal strengths were compared to the HI. EXPERIMENTALINVESTIGATION signal strength corresponding to the 0 dB link margin. The RF coverage was determined for proposed astronaut worksites. The experimental data were used to verify the antenna performance and to assess the validity of the computer simulation. A series of the Space Shuttle UHF antenna 11. COMPUTATIONALMETHOD near-field measurements based on the 1110th scale Space Shuttle mockup were conducted in the NASA Johnson Space The Geometrical Theory of Diffractions was used in the Center (JSC) Anechoic Chamber antenna test facility. simulations to take into account the multipath and blockage The Anechoic Chamber in the NASA Johnson Space Center effects from the Space Shuttle vehicle structures. has a l/lOth scale Orbiter mockup, as shown in Fig. 2, that was constructed for antenna testing. The l/lOth scale UHF antenna A. Assumptions was mounted inside the payload bay in this Orbiter mockup. It is assumed that the electromagnetic properties of the Space Since the entire test was performed with 1110th scale hardware, Shuttle structures are such that the surfaces are highly reflective the frequencies used for the testing were 10 times higher than the original frequency (e.g. 4.1565 GHz to model the center frequency of 0.41565 GHz). The Space Shuttle near-field test setup is shown in Fig. 3. The signal strengths underneath the Space Shuttle were measured for the UHF antenna in the Space Plate Blockage (16.375", y=O) Shuttle payload bay. I I

-50 hm 9 2 -60 0 -I m c "a -70 I I 3 8 x Measured Ex o Measured Ey -80 Computed Ex

-90 I -6 -3 3 6 Fig. 2. The Moth scale SSO mockup in NASMJSC Anechoic Chamber. 0 X (Inches)

Fig. 4. Measured and computed signal strength along x-axis underneath a flat plate.

Plate Blockage (16.375", x=O) 1

Fig. 3. Near-field measurement setup in NASMJSC Anechoic Chamber.

N. COMPUTER SIMULATION VALIDATION

The signal strengths along EVA paths underneath the Space Shuttle using the payload bay UHF antenna were computed using the GTD simulation tool. The computed electric fields o MeasuredEy were compared to the signal strength corresponding to the 0 dB H-Computed Ex H link margin. Experimental data were obtained fi-om the -Computed Ey NASNJSC Anechoic Chamber near-field measurements. The -90 I I I comparisons of measured and computed signal strength in the shadow region underneath a flat plate were shown in Figures 4 and 5. The comparisons of measured and computed signal Y (Inches) strength in the shadow region underneath the Space Shuttle were shown in Figures 6 and 7. Reasonable agreement between measured and computed results is observed. Fig. 5. Measured and computed signal strength along y-axis underneath a flat plate. and repair tasks. Data obtained fkom both the computer simulations and near-field measurements were analyzed. argin (Path at X=524") 25 The Space Shuttle payload bay UHF antenna transmit power is -3.4 dBW. The circuit loss is -2.1 dB. The UHF antenna 20 signal strengths (dBV/m) at 2 meters underneath the Space Shuttle Orbiter are shown in Fig. 8. The signal level on the 15 nm starboard side is higher than the signal level on the port side due 2 10 to the payload bay UHF antenna location. The data indicates .-c that the lowest signal levels are in the regions near the tail due to p5 the significant wing blockage in those areas. As expected, the 2 signal levels improve with increasing separation distance between the astronauts and the Orbiter underside [S-1 I]. yof 2 -5 SSO ant signal (dBV/m) at 2m underneath Space Shuttle -10 -70-65 0-65-60 -50-45 II -45--40 -40-35 -15 -15 -10 -5 0 5 10 15 Y (Meters)

Figure 6. Measured and computed signal strength along an EVA path underneath the Space Shuttle.

Link Margin (Path at X=956") 25

20

15 9 X (Inches) .-S g IO Fig. 8. Space Shuttle UHF antenna signal strengths (dBV/m) at 2 meters 2 underneath the Space Shuttle Orbiter. Y S 25 Computer simulations and experimental measurements were also performed for the International Space Station (ISS) UHF Space-to-Space Communications System (SSCS) Lab and P1 0 truss antennas. The ISS SSCS antennas are available and can be used for the EVA communications when the Space Shuttle is -5 docked to the Space Station. The ISS UHF antenna locations are -15 -10 -5 0 5 10 15 shown in Figs. 9 and 10. Y (Meters) The Space Station UHF Lab antenna transmit power is -3.9 dBW. The circuit loss is 4.2 dB. The Space Station UHF Lab antenna signal strengths (dBV/m) at 2 meters underneath the Fig. 7. Measured and computed signal strength along an EVA path underneath Space Shuttle are shown in Fig. 11. The signal level on the the Space Shuttle. Shuttle starboard side is much higher than anywhere else due to the ISS Lab antenna location. When the Shuttle is docked to Station, the ISS Lab antenna points generally toward the inside V. COMMUNICATIONPERFORMANCE EVALUATION of the open starboard payload bay door. The data indicates that the lowest signal levels are in the regions near the tail due to the The communications performance is evaluated for EVA significant wing blockage in those areas. The signal level is in communications when astronauts performing vehicle inspection general better for the ISS Lab antenna than for the SSO UHF antenna due to the ISS Lab antenna favorable location and pointing- direction. The ISS UHF Lab antenna is a good ISSLab ant signal (dBVlm) at 2m underneath Orbiter candidate as the primary antenna for EVA communications underneath Space Shuttle.

-720 -480 -240 3 Q) c 0:- 240 F 480

\. . . . . , , . .-, , , , , , , , , , , , , I ;-720

X (Inches) Fig. 11. Space Station Lab antenna signal strengths (dBV/m) at 2 meters underneath the Space Shuttle. Fig. 9. Front view of Space Station antenna locations. ISS PI ant signal (dBV/m) at 2m underneath Orbiter

-720 -480 -240 ‘;i a, .s 02- 240 7 480 731)

Fig. 10. Side view of Space Station antenna locations X (Inches) Fig. 12. Space Station P1 antenna signal strengths (dBV/m) at 2 meters The Space Station UHF P1 antenna transmit power is -3.9 underneath the Space Shuttle. dBW. The circuit loss is -8.4 dB. The ISS UHF P1 antenna signal strengths (dBV/m) at 2 meters underneath the Space Shuttle Orbiter are shown in Fig. 12. The signal level on the VI. CONCLUSION Shuttle port side is much higher than anywhere else due to the In this study, the communication performance underneath the ISS P1 antenna location. The signal level is in general better for Space Shuttle Orbiter using the Space Shuttle and Space Station the ISS P 1 antenna than for the SSO UHF antenna due to the ISS UHF antennas were analyzed. The shadowing effects &om the P1 antenna favorable location and pointing direction. The ISS Space Shuttle vehicle structures blocking the UHF antennas UHF P1 antenna is a good candidate as the secondary antenna were investigated. The computer simulation tool based on the for EVA communications underneath Space Shuttle. Geometrical Theory of Diffkaction method was used to compute the signal strengths. Based on the results obtained in this study, In the near field the generally has three RF coverage areas for the underside communications were orthogonal components with non-uniform magnitude. In order determined. to obtain the optimum communication performance, an occasional EVA astronaut maneuvering of one or two feet or a Good EVA astronaut communication coverage can be slightly different orientation of 10 or 20 degrees may be achieved by using the Space Station Lab antenna as the primary necessary. antenna and switching to the Space Station P1 truss antenna when necessary. The signal levels improve with increasing separation distance between the EVA astronauts and the Orbiter underside. The data indicate that the lowest signal levels are in the regions near the tail due to the significant wing blockage in those areas. The signal level is in general better for the Space Station Lab and P1 antennas than for the Space Shuttle payload bay antenna due to the Space Station LabP1 antenna favorable location and pointing direction. The Space Station Lab antenna provides better signal levels on the Shuttle port side and the Space Station P1 antenna provides better signal levels on the Shuttle starboard side.

It has been shown that a communication link is possible for many places underneath the Space Shuttle. The areas of poor link are in the deep shadow region and have Men identified by this work. An astronaut can possibly improve a poor lid condition by a re-orientation. An astronaut relay scenario using a second astronaut to relay messages can also improve the coverage area.

ACKNOWLEDGMENT

The authors wish to acknowledge the contributions fkom Wayne Cope of Lockheed Martin for implementation of the near-field measurements. The review and comments by William Culpepper of NASNJSC are appreciated.

REFERENCES

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